We learn it from a young age: solids hold their shapes; liquids flow. Physical states of matter are mutually exclusive. A solid occupies a particular position in space, its molecules fixed. A fluid assumes the shape of its container, its molecules in constant motion. But a so-called supersolid, a predicted phase of matter that forms only under extreme circumstances, doesn’t follow this idea of order. To describe supersolids is an exercise in contradictions. On the one hand, they form rigid crystalline structures. On the other, theory predicts that part of their mass also acts like a superfluid – a quantum phase of matter that flows like a liquid, but without viscosity. That combination lets supersolids do things that seem unfathomable to the humdrum, room-temperature, Newtonian world, like flow through themselves – without friction.

Although the Russian physicists Alexander Andreev and Ilya Liftshitz first predicted in 1969 that supersolids could form in helium close to absolute zero, definite proof has been hard to come by, and this elusive phase of matter has largely remained entrenched in the world of theory. That may have changed, though: two independent groups of researchers – one at the Massachusetts Institute of Technology (MIT) in the US, and the other at ETH Zurich in Switzerland – recently reported forming supersolids

Read more about the weirdness of supersolids here, in an article from Physics World.